As Clayney0 said NP isn't really the best for reaver, however, if your having trouble killing the bombs it can be quite nice.
Here is a trinket guide that he might find useful, it was written by Valithyria on the DK Forums
You want to have the following trinkets: Discordant Chorus, Reaper's Harvest, Unending Hunger, and Vial of Convulsive Shadows.
Generally speaking you want to use Unending Hunger + Vial of Convulsive Shadows for ST/cleave fights, and Unending Hunger + X for AoE fights (X depending on your circumstances, but mainly Reaper's Harvest or Vial, sometimes DC) .
Hellfire Assault: Unending Hunger, Discordant Chorus, and Vial of Convulsive Shadows are all nice.
Iron Reaver: Unending Hunger + Vial of Convulsive Shadows
Kormrok: Unending Hunger + Discordant Chorus (or Reaper's Harvest if you contribute most of the damage to the hands)
Kilrogg: Unending Hunger + Vial of Convulsive Shadows
Hellfire High Council: Unending Hunger + Reaper's Harvest (or Vial of Convulsive Shadows if playing Breath)
Gorefiend: Unending Hunger + Reaper's Harvest
Iskar: Unending Hunger + Reaper's Harvest/Vial
Socrethar: Unending Hunger + Vial of Convulsive Shadows
Tyrant Velhari: Unending Hunger + Vial of Convulsive Shadows
Fel Lord Zakuun: Unending Hunger + Vial of Convulsive Shadows
Xhul'horac: Unending Hunger + Reaper's Harvest/Discordant Chorus
Mannoroth: Unending Hunger + Reaper's Harvest/Vial
Archimonde: Unending Hunger/Vial + Reaper's Harvest (on mythic, for heroic Unending Hunger + Vial with BoS)
Reaper's Harvest is dispensable on all of these fights except Council if your raid has lots of burst AoE - it's only a solid gain if Wandering Plague is ~9% of your damage.
Going through the logs in a bit more detail:
Reaver:
* Seems to be running Reapers Harvest (Class trinket) and Pebble? Reapers harvest is TERRIBLE for single target, worse than even the leech/speed/avoidance trinket. He seems to also have a UeH and should be using this and possibly a vial if he has one instead.
* Could have gotten a few more soul reapers.
* Cast Icy Touch quite a bit, not really sure why, Maybe trying to get extra damage on the boss during blitz?
Kormrok
* Still running reapers, not the greatest, but not quite as bad as on reaver.
* More Icy Touches and some Death Strikes. Not sure if he was using them to save himself or not.
* More soul reapers.
Gorefiend:
* Guessing he is your soul breaker? Seems like you have quite a few assigned to it so he won't blow up the meters.
* He loves him some icy touches, not really a point in casting it.
* 40 horn of winter casts, it not longer gives RP.
Mostly he just needs to learn what trinkets to use when. On Gorefiend he's not going to really destroy the meters unless he does almost all the soul damage or goes full on BoS play style for massive feast of souls damage. Random icy-touches are bas and are hurting him some also. Learning BoS playstyle is important and will help him on future bosses.
One other thing I only recently learned, if you have UeH procced you can spam glpyhed death grip into the boss to make it stack quicker.